Hexachlorobenzene (HCB) exposure has been shown to induce hyperparathyroidism and osteosclerosis in rats. Experiments were undertaken to investigate the effects of HCB on femur morphometry as well as breaking strength. Fischer 344 rats were dosed 5 days/wk for 15 wks with 0, 0.1, 1, 10 or 25 mg HCB/kg body weight. Hyperparathyroidism was produced in the two higher dose groups as reported previously (Andrews et al., 1988). Femur weight was significantly increased in the rats receiving 0.1, 1 and 25 mg HCB/kg body weight wheras density was increased significantly at 1, 10 and 25 mg HCB/kg dose levels. Bone strength was also significantly increased at the three higher dose levels. Bone flexibility was significantly increased at the 0.1 mg HCB dose level. Cross sectional area of the midpoint of the femur was significantly increased at the 1 mg/kg HCB dose level. Cortical area, as well as the proportion of the total area of the bone which the cortex occupied, were significantly increased at the three higher dose levels. Medullary cavity area was significantly increased at the 0.1 mg/kg dose level but significantly decreased at the two higher dose levels of HCB.